The present invention generally relates to the production of single crystal silicon, and more particularly to a method and apparatus for preparing a molten silicon melt from polycrystalline silicon.
Most single crystal silicon used for microelectronic circuit fabrication is prepared by the Czochralski (CZ) process. In this process, a single crystal silicon ingot is produced by melting polycrystalline silicon in a crucible, dipping a seed crystal into the molten silicon, withdrawing the seed crystal in a manner sufficient to achieve the diameter desired for the ingot and growing the single crystal at that diameter. The polycrystalline silicon melted to form the molten silicon is typically irregularly shaped chunk polycrystalline silicon prepared by the Siemens process or, alternatively, free-flowing, generally spherically-shaped granular polycrystalline silicon, typically prepared by a fluidized-bed reaction process.
The initial charging of chunk type polycrystalline silicon into the crucible and the melting thereof can introduce undesirable impurities and defects into the single crystal silicon ingot. For example, when a crucible is initially charged entirely with chunk polycrystalline silicon, the edges of the chunks under the load of a full charge can scratch and gouge the crucible wall, resulting in a damaged crucible and in particles of crucible floating on or being suspended in the silicon melt. These impurities significantly increase the likelihood of dislocations forming within the single crystal, and decrease the dislocation-free single crystal production yields and throughput. Careful arrangement of the chunk-polycrystalline silicon during the initial loading can minimize the thermal stresses. As melting proceeds, however, the charge can shift or the lower portion of the chunk-polycrystalline silicon can melt away and leave either a xe2x80x9changerxe2x80x9d of unmelted material stuck to the crucible wall above the melt or a xe2x80x9cbridgexe2x80x9d of unmelted material bridging between opposing sides of the crucible wall over the melt. When the charge shifts or a hanger or bridge collapses, it may splatter molten silicon and/or cause mechanical stress damage to the crucible. Additionally, initial loadings of 100% chunk-polycrystalline silicon limits the volume of material which can be charged due to the poor packing densities of such chunk materials. The volume limitations directly impact single crystal throughput.
Problems similarly exist when a CZ crucible is initially charged entirely with granular polycrystalline silicon. Large amounts of power are required to melt the granular polycrystalline silicon due to its low thermal conductivity. The thermal stress induced in the crucible by exposure to such high meltdown-power can cause distortion of the crucible and particles of the crucible to be loosened and suspended in the melt. Like the mechanical stresses, these thermal stresses result in reduced crystal throughput. Other problems associated with initial charges comprising 100% granular polycrystalline silicon are disclosed below with respect to the present invention. Finally, although initial loadings of granular polycrystalline silicon may be volumetrically larger than that of 100% chunk polycrystalline silicon, they typically do not result in higher overall throughput, because the degree of thermal stress on the crucible increases with the size of initial loading.
Whether the crucible is initially loaded with chunk or granular polycrystalline silicon, in many processes it is desirable to add polycrystalline silicon to the melt with a feeding/metering system to increase the quantity of molten silicon in the crucible. The use of such additional loadings of charge-up polycrystalline silicon is known for batch, semi-continuous or continuous process systems. In the batch system, for example, additional polycrystalline silicon may be loaded into the existing melt to achieve full crucible capacity in light of the decrease in volume after the initial polycrystalline silicon charge is melted.
To this end, co-assigned U.S. Pat. No. 5,588,993 discloses a method for preparing molten silicon from polycrystalline silicon charge in which polycrystalline silicon, preferably chunk polycrystalline silicon, is loaded into a crucible and partially melted to form molten silicon and unmelted silicon having an upper surface extending above the molten silicon (otherwise referred to as the island of unmelted silicon). Granular polycrystalline silicon is fed onto the exposed, unmelted silicon until a desired total amount of polycrystalline silicon is loaded in the crucible. The granular polycrystalline silicon and the unmelted silicon are then fully melted to form a molten silicon melt. This method results in improved zero defect yield, throughput and mean hot cycle times during the production of single crystal silicon ingots.
However, successful performance of this process requires the operator to manually view and control, based on observation, the size of the island of unmelted silicon in the crucible as the granular polycrystalline silicon is fed onto the unmelted silicon. The size of the island is controlled by varying the side and bottom heater power and the feed rate at which granular polycrystalline silicon is fed onto the island. For example, if the island becomes too large, bridging of the island to the crucible wall becomes a concern and the operator decreases the feed rate. If the island becomes too small, there is a risk that granular polycrystalline will undesirably fall directly into the melt and the operator accordingly increases the feed rate. This practice can result in substantial variability in crystal quality due to differences in operators and the extent of operator attention during the melting process.
Among the several objects and features of the present invention may be noted the provision of a method and apparatus for preparing molten silicon melt from polycrystalline silicon in a crystal pulling apparatus; the provision of such a method and apparatus which increases consistency of crystal quality; the provision of such a method and apparatus which automatically control the rate at which polycrystalline silicon is fed into a crucible of the crystal pulling apparatus; the provision of such a method and apparatus which increase throughput of the crystal pulling apparatus; and the provision of such a method which can be carried out efficiently and economically and such apparatus which is economically feasible and commercially practical.
In general, a method of the present invention for preparing molten silicon melt from polycrystalline silicon in a crystal pulling apparatus comprises loading polycrystalline silicon into a crucible. The amount of polycrystalline silicon loaded into the crucible is substantially less than a predetermined total amount of polycrystalline silicon to be melted in the crucible. The crucible is then heated to melt down the polycrystalline silicon in the crucible to form a partially melted charge in the crucible. The partially melted charge comprises molten silicon having an upper surface and an island of unmelted polycrystalline silicon exposed above the upper surface of the molten silicon. Granular polycrystalline silicon is fed from a feeder onto the island of unmelted polycrystalline silicon in the crucible until the predetermined total amount of polycrystalline silicon has been loaded into the crucible. The position of the island of unmelted polycrystalline silicon relative to the side wall of the crucible is determined electronically, with this step being conducted as granular-polycrystalline silicon is fed onto the island of unmelted polycrystalline silicon in the crucible. The feed rate at which granular polycrystalline silicon is fed from the feeder onto the island of unmelted polycrystalline silicon is controlled in response to the determined position of the island of unmelted polycrystalline silicon relative to the crucible side wall at the upper surface of the molten silicon.
In another embodiment, apparatus of the present invention for preparing molten silicon melt from polycrystalline silicon in a crystal puller used for growing monocrystalline silicon ingots according to the Czochralski method comprises a heater for heating a crucible of the crystal puller to melt down polycrystalline silicon in the crucible to form a partially melted charge in the crucible comprising molten silicon having an upper surface in the crucible and an island of unmelted polycrystalline silicon exposed above the upper surface of the molten silicon. A feeder is adapted for feeding granular-polycrystalline silicon onto the island of unmelted polycrystalline silicon in the crucible. A camera is arranged for generating a video image signal of at least a portion of the crucible, the molten silicon and the island of unmelted polycrystalline silicon. A vision system is provided and is capable of electronically determining from the video image signal the position of the island of unmelted polycrystalline silicon relative to the crucible side wall. The feeder is in electrical communication with the vision system and is responsive to the determined position of the island of unmelted polycrystalline silicon relative to the crucible side wall to control the feed rate at which granular-polycrystalline silicon is fed from the feeder onto the island of unmelted polycrystalline silicon.